Disc drives of the type known as "Winchester" disc drives, or hard disc drives, are well known in the industry. Such disc drives magnetically record digital data on a plurality of circular, concentric data tracks on the surfaces of one or more rigid discs. The discs are typically mounted for rotation on the hub of a brushless DC spindle motor. In disc drives of the current generation, the spindle motor rotates the discs at speeds of up to 10,000 RPM.
Data are recorded to and retrieved from the discs by an array of vertically aligned read/write head assemblies, or heads, which are controllably moved from track to track by an actuator assembly. The read/write head assemblies typically consist of an electromagnetic transducer carried on an air bearing slider. This slider acts in a cooperative hydrodynamic relationship with a thin layer of air dragged along by the spinning discs to fly the head assembly in a closely spaced relationship to the disc surface. In order to maintain the proper flying relationship between the head assemblies and the discs, the head assemblies are attached to and supported by head suspensions or flexures.
The actuator assembly used to move the heads from track to track has assumed many forms historically, with most disc drives of the current generation incorporating an actuator of the type referred to as a rotary voice coil actuator. A typical rotary voice coil actuator consists of a pivot shaft fixedly attached to the disc drive housing base member closely adjacent the outer diameter of the discs. The pivot shaft is mounted such that its central axis is normal to the plane of rotation of the discs. An actuator bearing housing is mounted to the pivot shaft by an arrangement of precision ball bearing assemblies, and supports a flat coil which is suspended in the magnetic field of an array of permanent magnets, which are fixedly mounted to the disc drive housing base member. On the side of the actuator bearing housing opposite to the coil, the actuator bearing housing also typically includes a plurality of vertically aligned, radially extending actuator head mounting arms, to which the head suspensions mentioned above are mounted. When controlled DC current is applied to the coil, a magnetic field is formed surrounding the coil which interacts with the magnetic field of the permanent magnets to rotate the actuator bearing housing, with the attached head suspensions and head assemblies, in accordance with the well-known Lorentz relationship. As the actuator bearing housing rotates, the heads are moved radially across the data tracks along an arcuate path.
When a disc drive is in an unpowered condition, it is common industry practice to move the heads to a predetermined park position and to latch the actuator at the park position in order to prevent uncontrolled contact between the data heads and the data recording areas on the disc surfaces. There are two general types of "head parking": 1) contact start/stop, in which the heads are brought to rest on a specially reserved portion of the discs near the inner diameter, and; 2) ramp parking drives, in which the heads are lifted away from the disc surfaces by ramp structures positioned closely adjacent the outer diameter of the discs.
With either type of parking scheme, some sort of latch is necessary to hold the actuator at the park location in the presence of mechanical shocks applied to the disc drive. The simplest types of latching mechanisms are sometimes grouped together as "passive latch/unlatch" devices, and commonly include magnetic contact between a latch mechanism fixedly mounted to the disc drive housing and a magnetic contact feature mounted on the moving portion of the actuator. During latching operation, the only action needed is to move the actuator to the park position, at which time the magnetic attraction between the latch mechanism and the contact feature causes the actuator to be latched. Unlatching is accomplished by using the actuator motor to move the actuator away from the latch position with sufficient power to overcome the magnetic attraction.
Such latches are simple and inexpensive to implement, but are commonly not capable of withstanding the amounts applied mechanical shocks specified for disc drives of the current and future generations. For instance, disc drives are being specified to withstand radial shock accelerations on the order of 10 to 30 thousand radians per second.sup.2 applied over time intervals of 0.001 to 0.003 seconds. While it is relatively simple to design magnetic latches capable of withstanding these types of shocks, the magnetic attraction necessary becomes relatively large, and the power to overcome the magnetic attraction for unlatching of the actuator is often not available in the actuator motor, particularly in disc drives of the present small physical dimensions.
It would therefor be desirable to provide a passive magnetic latching mechanism which is capable of withstanding large amounts of applied mechanical shocks without unintentional unlatching and which would still require a relatively small force to unlatch when such unlatching is intended.